The study is designed to collect data of the performance and confirm the safety of the DX (Diagnostic eXtension) functionality in the Lumax 640/470 HF-T in patients with permanent atrial fibrillation and CRT-D indication according to current ESC guidelines. The DX functionality is a feature, which can be activated in the Lumax 640/740 HF-T when connected to the LinoxSMART S DX right ventricular lead. The combination of these devices enable atrial sensing via a sensing dipole in the ventricular lead and therefore reduces the number of implanted leads without sacrificing atrial information. Atrial pacing can not be provided but is not needed in patients with permanent atrial fibrillation.